Crystals of chicken skeletal muscle troponin-C (TN-C) have been obtained which produce high resolution diffraction data. These are the first crystals of a muscle contractile protein which are capable of allowing a high resolution structure determination. Crystals of chicken TN-C will be subjected to x-ray diffraction analysis to develop a high resolution structure. The path of the peptide backbone will be determined and compared with the folding of parvalbumin (a low molecular weight calcium binding protein in the cytoplasm of muscle). Attepts will be made to crystallize cardiac TN-C, the other troponin subunits, and the troponin complex. Work toward crystallization of TN-C in other calcium states will be initiated. The protein-protein contact areas among the thin filament regulatory protein complex will be mapped using the differential chemical modification technique. The effect of calcium on the reactivities of the surface amino acid side chains will be examined. This combined crystallographic and biochemical study will eventually define the three-dimensional structure of the proteins which act as the calcium switch in muscle contraction and define their intra-and inter-molecular organization.